Zeolites are crystalline tectosilicates. Their structures typically consist of assemblies of TO.sub.4 tetrahedra forming a three dimensional framework by sharing of the oxygen atoms. In zeolites of the aluminosilicate type, which are the most common, T represents tetravalent silicon as well as trivalent aluminum. The cavities and channels of this framework are of molecular dimension and collect cations, compensating the charge deficit associated with the presence of trivalent aluminum in the tetrahedra. Trivalent elements such as gallium or boron can be substituted for the aluminum.
In general, the composition of zeolites can be represented by the empirical formula M.sub.2/n O.multidot.Y.sub.2 O.sub.3 .multidot.xZ0.sub.2 in the dehydrated and calcined state. Z and Y respectively represent the tetravalent and trivalent elements of the TO.sub.4 tetrahedra. Typically, Z is Si and Y is Al. M represents an electropositive element of valence n such as an alkali or alkaline earth metal, constituting the compensating cations. The value of x may range in theory from 2 to infinity, in which case the zeolite is a silica (silicalite).
Each type of zeolite has a distinct porous structure. Examples of types of zeolites having different three dimensional arrangements of their framework elements include ZSM-5 (MFI), zeolite beta, zeolite A, and so forth. The variation in pore size and shape from one type to another causes changes in the absorbent properties. Only molecules with certain sizes and shapes are capable of entering the pores of a particular zeolite. The chemical composition along with, in particular, the nature of the elements present in the TO.sub.4 tetrahedra and the nature of the exchangeable compensating cations are also important factors affecting the absorptive selectivity and especially the catalytic properties of these products. Zeolites are consequently used as catalysts or catalyst supports in the cracking, reforming, and modification of hydrocarbons and the synthesis of various organic compounds.
For example, molecular sieves containing titaniun atoms isomorphously substituted for a portion of the silicon atoms in their framework lattice have in recent years been found to be highly active and useful catalysts for the epoxidation of olefins using hydrogen peroxide. See, for example, U.S. Pat. Nos. 4,833,260 and 5,453,511.
The substitution of different transition metals into the framework structures of molecular sieves is not straightforward, however, and often can only be successfully accomplished through very careful selection of reactants and reaction conditions. The preparation of transition metal-containing molecular sieves remains a highly uncertain and unpredictable art. For instance, while European Pat. Pub. No. 77,522 claimed the preparation of titano-aluminosilicates having a pentasil (ZSM-5) structure, later workers (Skeels et al., U.S. Pat. No. 5,098,687) demonstrated that the titanium atoms in the materials obtained were not actually present in the form of framework tetrahedral oxides.
To date, there have been few reports of niobium-containing molecular sieves in the literature. European Pat. Pub. No. 178,723 disclosed catalyst compositions for catalytic cracking of hydrocarbons comprised of porous matrix material containing crystalline aluminosolicates and a niobium component. However, it is clear from the description provided that the niobium in said catalyst compositions is merely impregnated or supported on the crystalline aluminosilicate and is not incorporated in the framework structure of the zeolite itself. Japanese Kokai JP 04-349,115 teaches crystalline silicates having the chemical composition in dehydrated form: EQU (0.1-2.0)R.sub.2.0)R.sub.2/n O.multidot.[aM.sub.2 O.sub.3 .multidot.bAl.sub.2 O.sub.3 ].multidot.ySiO.sub.2
wherein R is .gtoreq.1 monovalent or divalent ions, n is the valence of R, M can be Nb or another metal, a+b=1,a&gt;1,b.gtoreq.0,y.gtoreq.12. Such materials thus must necessarily contain relatively high levels of exchangeable cations (e.g., alkali metal oxide or alkaline earth metal oxide cations). There is no suggestion that such crystalline silicates could effectively catalyze the epoxidation of olefins.